This invention relates to an optical fiber of the step-index type in which the core material is an inorganic glass and the cladding material a vinylidene fluoride base copolymer.
There is no doubt about a very important role of optical communications in the recent and future highly information-oriented society. A key element in an optical communication system is optical fiber. In the light communication systems already put to practical use and particularly in the long distance and high capacity communication systems, quartz or silica base glass is the most prevailing material for optical fibers mainly because of low transmission loss in such glass.
Recently some kinds of organic plastics optical fibers have also been developed mostly by using a methacrylic resin as the core material. Advantages of plastics optical fibers reside in the ease of fabrication and processing and the feasibility for considerable enlargement of optical fiber diameters. However, plastics optical fibers are, and will probably remain inferior to inorganic glass optical fibers in respect of transmission loss. Therefore, practical applications of plastics optical fibers have been limited to short distance transmissions mostly for display purposes.
In view of such merits and demerits of inorganic optical fibers and organic optical fibers, increasing interest has been attached to the third category of optical fibers which are of a composite type using a silica base glass as the core material and an organic polymer as the cladding material. The composite type optical fibers are lower in transmission loss than the organic optical fibers and, besides, might be less costly. Accordingly the composite type optical fibers are expected to have wide applications to short and medium distance communications particularly in the fields of office automation and factory automation.
In the composite type optical fibers quartz glass will be preferred as the core material because of its excellence in light transmittance. Since quartz glass is relatively low in refractive index (about 1.458), the organic polymer as the cladding material must be selected among limited kinds of synthetic resins that have low refractive indices and is usually selected from silicone resins and fluorine-containing resins. The cladding of a quartz glass core with an organic polymer is accomplished by an extrusion method or a solution coating method depending on the properties of the selected polymer. For example, Japanese patent application primary publication No. 51-52849 (1976) shows an optical fiber of the step-index type in which the core material is either quartz glass or an inorganic optical glass and the cladding material a copolymer of tetrafluoroethylene with vinylidene fluoride. This cladding material is suited to both extrusion and solution coating.
For a practicable optical fiber of the composite type the following items are required of the organic cladding material: (1) to be sufficiently low in refractive index, (2) to be high in transparency and preferably transmissive to visible light, (3) to be good in adhesion to the core glass, (4) to be low in crystallinity, (5) to be higher than about 100.degree. C. in thermosoftening temperature, (6) to be capable of providing a tough and flexible coating film, (7) to be chemically stable, (8) to be high in weatherability and (9) to be obtained at low cost.
Since large demands for optical fibers for short and medium distance communications are expected, it is also a matter of primary importance for the composite type optical fibers to be fabricated by a simple, efficient and economical cladding method. In this regard, a solution coating method is advantageous over an extrusion method because the core filament drawing operation and the cladding operation can be performed successively at the same station and also because the thickness of the cladding can be reduced. For a practical solution coating operation it is desirable that the cladding material be well soluble in a commonplace solvent that has a relatively low boiling point and is free of or weak in toxicity.
Another matter of importance for the composite type optical fibers is that the cladding layer does not need to be further coated with a protective layer. That is, the cladding layer should be untacky and sufficiently strong. In general silicone resins do not meet this requirement. Besides, cladding with a silicone resin is relatively low in productivity even though a solution coating method can be employed because a curing step must be incorporated.